During its commissioning phase, LST-1 identified a slight excess in gamma-ray flux while monitoring GRB 221009A, offering fresh evidence in support of theoretical models proposing structured jets with layered emission zones. This contrasts with older models depicting jets as uniformly narrow beams.
Gamma-ray bursts are some of the Universe's most extreme events, emitting in seconds what the Sun produces across its entire life. They feature an initial, brief outburst followed by a long-lasting afterglow. GRBs fall into two types: long ones, typically linked to powerful supernovae, and short ones, likely caused by neutron star mergers. Due to their transient nature and weakened gamma rays over distance, detecting GRBs at very high energies remains difficult.
The long-duration GRB 221009A was spotted on 9 October 2022 by space telescopes including NASA's Fermi and Swift. Its exceptional brightness overwhelmed many instruments, prompting a global flurry of follow-up observations.
Just 1.33 days after the initial flash, LST-1 began tracking the burst and continued over a 20-day period. Although the gamma-ray excess it observed did not meet the threshold for an official detection, it enabled the team to define tight upper limits on the emitted very high-energy gamma-ray flux. These constraints sharpen the field's ability to test competing jet models.
LST-1 data lend strong support to the hypothesis that GRB 221009A involved a complex jet with a high-speed central core encased in a slower outer sheath, departing from earlier "top-hat" jet assumptions. This helps clarify jet formation processes and the nature of GRB central engines.
Remarkably, the telescope operated under bright moonlight, a scenario typically unfavorable for Cherenkov telescopes due to light sensitivity. LST-1's enhanced design permitted observations despite this challenge, marking the first time the instrument gathered data under such conditions.
These achievements underscore the potential of CTAO's next-generation telescopes to advance high-energy astrophysics. Construction of three additional LSTs at the La Palma site is underway, while the CTAO-South site in Chile is also in development. Intermediate telescope arrays in both hemispheres will soon enhance detection capability, bolstered by automated alert systems that enable faster responses to cosmic transients.
The CTAO LST Collaboration, comprising over 400 experts from 67 institutes in 11 nations, is tasked with developing the LST systems for CTAO as an In-Kind Contributor.
Research Report:GRB 221009A: Observations with LST-1 of CTAO and Implications for Structured Jets in Long Gamma-Ray Bursts
Related Links
Cherenkov Telescope Array
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